This invention relates to DC to DC converters in which converter switching elements turn on and off when current through them is zero. In this class of converters, the output voltage is usually determined by either the current into a resonating capacitor in series with the transformer of the converter, or the voltage level across the series resonating capacitor. The values of the capacitor voltage and current vary as the input voltage varies causing the output voltage of the converter to also vary. To control the output voltage of the converter, the frequency at which the switching devices operate is varied. This complicates the control circuitry of the converter. Additionally, a more complicated DC output voltage filter is required. Further description of this class of converters can be found in U.S. Pat. No. 4,785,387, issued to Fred Lee.